A volcanic eruption hurls ash particles and gas high into the sky. Once airborne, these substances circle the globe, scattering sunlight more and giving sunsets a redder color. The effect only lasts for a few years—but paintings by gifted artists can preserve it for centuries. And give us clues about past air quality.

Between the years 1500 and 2000, some 50 major volcanic eruptions occurred all over the world. Meanwhile, artists painted thousands of sunsets.

To see if such paintings could reveal atmospheric info, researchers analyzed high-quality digital photos of the art for the balance of red and green colors along the horizon. Based on the red-green ratio, they calculated the amount of particles in the atmosphere.